The present disclosure relates generally to subterranean treatment operations, and more particularly to methods and systems for evaluating and treating previously-fractured subterranean formations.
Hydrocarbon-producing wells are often stimulated by hydraulic fracturing operations, wherein a fracturing fluid is introduced into a hydrocarbon-producing zone within a subterranean formation at a hydraulic pressure sufficient to create or enhance at least one fracture therein. A fracture typically has a narrow opening that extends laterally from the well. To prevent such opening from closing completely when the fracturing pressure is relieved, the fracturing fluid typically carries a granular or particulate material, referred to as “proppant,” into the opening of the fracture. This material generally remains in the fracture after the fracturing process is finished, and serves to hold apart the separated earthen walls of the formation, thereby keeping the fracture open and enhancing flow paths through which hydrocarbons from the formation can flow into the well bore at increased rates relative to the flow rates through the unfractured formation. FIG. 1 illustrates an example of a proppant-filled fracture in a subterranean formation. FIG. 2 illustrates an example of fluid flowing through a fracture in a subterranean formation into a well bore.
Generally, designers of fracturing operations have assumed uniform fracture conductivity. However, some prior publications have pointed out that loss of fracture conductivity near the well bore may significantly adversely impact the productivity of a fractured well bore. This may be particularly true in cases where transverse fractures are created that intersect a horizontal well, or a horizontal portion of a well bore.
It has been found, however, that most fractures do not have a uniform conductivity. In some instances, the conductivity of a fracture may be varied intentionally, as in cases where an operator may desire to have higher conductivity and/or stronger proppant near the well bore. In some cases, an operator may desire to prevent backflow of proppant by placing, in the near-well-bore area, a specially designed proppant having a different conductivity and/or physical properties than that of the proppant used for the majority of the fracturing operation. In other instances, the conductivity of the fracture may vary as a result of the fracturing process, as in cases where the fracture propagates across multiple formations with different properties, which may cause the conductivity of the fracture to vary in the vertical direction as well as the horizontal direction. It is not uncommon for fracture conductivity in the near-well-bore area to decline significantly with time and adversely affect the performance of the fractured well.
Impairment or loss of fracture conductivity may occur for a variety of reasons. For example, weakening of the proppant over time may impair fracture conductivity. As another example, fracture conductivity may be impaired by increasing closure pressure that may be caused by continued depletion of hydrocarbons in the formation as the well is produced. Fracture tortuosity also may lead to impairment of conductivity in some cases. Additionally, in some cases proppant may be over-displaced in certain regions of the fracture, which may reduce the amount of proppant that is deposited in the near-well-bore area. FIG. 3 illustrates an example of a subterranean fracture having a damaged area.
The effect of fracture conductivity damage may be greatly pronounced in previously-fractured horizontal wells. The performance of transverse fractures having finite conductivity has only recently been studied. Transverse fractures in a horizontal well differ from a vertically fractured well, in that the fluid in the fracture for a horizontal well converges radially toward the well bore as illustrated in FIGS. 4 and 5. FIGS. 4 and 5 illustrate different views of the convergence of fluid inside an exemplary transverse fracture intersecting an exemplary horizontal well bore. Such convergence may yield a flow regime different than the flow regime that may be expected when a vertical well is fractured.
Conventionally, operators evaluating well bores that are suspected to suffer from lost or impaired fracture conductivity have lacked means to differentiate between the loss of conductivity over the entire length of the fracture, and the loss of conductivity in only the near-well-bore area. For example, a refracture-candidate diagnostic regime has been proposed that comprises, among other things, a brief injection of fluid above the fracture initiation and propagation pressure for a formation, followed by an extended period of monitoring the decrease in pressure (e.g., “pressure-falloff”). The pressure falloff data is then plotted on a variable-storage, constant-rate drawdown type curve for a well producing from one or more vertical fractures in an infinite-acting reservoir. This diagnostic regime may determine, among other things, whether a pre-existing fracture exists, as well as whether such pre-existing fracture may be damaged. This regime also may provide estimates of, among other things, the fracture conductivity, the effective fracture half-length, the reservoir transmissibility, and the average reservoir pressure. However, where a pre-existing fracture exists, and is in damaged condition, conventional diagnostic regimes such as the one described above fail to diagnose whether such damage resides in the vicinity of the well bore, or whether the damage exists over a significant length of the fracture. This is problematic, because if an estimation of damage to a fracture leads an operator to conclude (perhaps erroneously) that conductivity has been lost over a significant length of the fracture, the operator may deem further remedial operations to be unjustified. However, if an operator estimating damage to a fracture could accurately determine that the loss of conductivity was confined to only about the near-well-bore area, the operator may justify a remedial operation that restores conductivity in or about the near well bore region.